Period 3: Sodium to Argon
Variations in physical properties You will recall that a large number of chemical and physical properties vary periodically with their atomic number. This idea of periodicity is the basis of the periodic table. For this section of the syllabus, we will study the trends and gradations in properties across period 3, sodium to argon. At the end of this section you will be able to relate the periodic trends in atomic properties such as ionisation energy and atomic radii to structure and bonding of the elements. The structure and bonding of the elements is in turn related to the physical properties of the elements such as melting points and densities. Atomic Radii As you move across the period from sodium to argon, the atomic radius decreases (Figure 1). The atomic radii shown in figure 1 are covalent radii. The covalent radius is defined as half the distance between two covalently bonded atoms. Other atomic radii may be be measured such as Van der Waals radii and metallic radii, however, when chemists refer to atomic radii, they usually mean covalent radii. But why does the atomic radius decrease as we go across the period? As we move across the period, electrons are being added to the same shell at about the same distance from the nucleus 1. Protons are also being added as we go across the period. The outer or valence electrons are pulled closer towards the nucleus as the positive charge in the nucleus increases resulting in a decrease in the atomic radius across the period. Electronegativity Electronegativity may be defined as the tendency of an atom to attract electrons Atoms may be ranked by their power to attract electrons in their chemical interactions by a numerical measure based on a scale developed by Linus Pauling (Figure 2). ''What trend is observed for electronegativities across a period? ''Notice that the electronegativities increase across period 3, from sodium to chlorine. The most electronegative elements are the non-metals, while the least electronegative elements are the metals. This was expected, because as you move across the period the effective nuclear charge increases and the atom has anincreasing ability to attract electrons. Density When comparing the densities of the elemnts across the period, we need to take into account the factors that affect the density such as atomic mass, atomic radius and crystal structure. Since each of these factors vary from one element to the next we expect to see a periodic trend in density. As you move from left to right across period 3, there is an increase in the density until a maximum (at Si in Group IV) and begins to decrease moving towards Ar. If you recall the density of a substance is its mass per unit volume. The atomic radius determines the volume of the elements and the crystal structure tells you how closely packed the atoms gan element are to each other. Reactions of the elements with oxygen, chlorine and water References #Hill, G., Holman, J. (2000) Chemistry in Context, 5th Ed UK: Thomas Nelson & Sons Ltd.